1. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People's Republic of China
2. Department of Electrical and Computer Engineering, Florida International University, Miami, Florida 33174, USA
3. Department of Physics, Beijing Normal University, Beijing 100875, People's Republic of China
4. EECS, University of California-Berkeley, Berkeley, California 94720, USA